Organic light-emitting diodes (OLEDs, also known as organic light-emitting devices) are generally anticipated to overtake liquid crystal displays (LCDs) as the preferred display technology. This is expected because OLEDs enjoy a number of practical advantages over LCDs. Some of the most significant advantages include: 1) OLEDs have a brighter image that can be viewed from wider angles; 2) elimination of backlight required in LCDs lowers cost, increases reliability, and improves image intensity range, contrast, and consistency over the viewing area; 3) OLEDs require less power for equivalent image quality; 4) OLEDs are potentially cheaper to manufacture, requiring fewer materials and roughly half the number of manufacturing steps; and 5) OLEDs produce a wider spectrum of colors. However, OLED displays and their components known as OLED structures, which constitute subpixels of the display, are more currently difficult and costly to manufacture than LCD displays. It is a continuing focus of the industry to increase throughput in an effort to lower the cost of OLED manufacturing.
Conventional OLED display devices are built on glass substrates such that a two-dimensional OLED array for image manifestation is formed. The basic OLED cell structure includes of a stack of thin organic layers sandwiched between an array of anodes and a common metallic cathode. The organic layers commonly comprise a hole-transporting layer (HTL), a light-emitting layer (LEL), and an electron-transporting layer (ETL). When an appropriate voltage is applied to the cell, the injected holes and electrons recombine in the LEL near the LEL-HTL interface to produce light (electroluminescence).
The LEL within a color OLED display device most commonly includes three different types of fluorescent or phosphorescent molecules that are repeated through the LEL. Red, green, and blue regions, or subpixels, are formed throughout the LEL during the manufacturing process to provide a two-dimensional array of pixels. Each of the red, green, and blue subpixel sets undergoes a separate patterned deposition, typically by evaporating a linear source through a shadow mask. Shadow masking is a well known technology, yet it is limited in both the precision of its deposition pattern, and the pattern's fill factor or aperture ratio; thus, incorporating shadow masking into OLED manufacturing limits the achievable sharpness and resolution of the resultant display. Radiation thermal transfer (RTT) promises significant advantages including a more precise deposition pattern and higher aperture ratio; however, it has proved challenging to adapt RTT to a high-throughput manufacturing line.
During RTT, a donor sheet having the desired organic material is placed into close proximity to the OLED substrate in a vacuum chamber. A source of radiation, such as a laser, impinges upon the donor sheet through a clear support to the donor sheet and is absorbed within a light-absorbing layer contained atop the support. The conversion of the radiation energy to heat sublimates the organic material that forms the top layer of the donor sheet and thereby transfers the organic material in a sharply defined subpixel pattern to the OLED substrate.
The quality of the RTT process is dependent on several key process and product parameters. Real-time knowledge of how these process and product parameters vary during manufacturing is an important aspect in making the RTT process repeatable and cost effective. What is needed is a way to measure key process and product parameters in-situ.
U.S. Pat. No. 6,485,884 provides a method for patterning oriented materials to make OLED display devices, and also provides donor sheets for use with the method, as well as methods for making the donor sheets. However, U.S. Pat. No. 6,485,884 patent fails to provide a system that enables in-situ monitoring of process and product parameters. To date, however, in-situ monitoring of process and product parameters has not been adapted to RTT.